Chemical properties of alkanes

Alkanes are quite inert substances with highly stable nature. Their inactiveness has been explained as:

(i) In alkanes all the C-C & C-H bonds being stonger sigma bonds and are not influenced by acids, alkalies, oxidants under ordinary conditions.

(ii) The C-C (completely non polar) & C-H (weak polar) bonds in alkanes- are practically non polar because of small electronegativity difference in C (2.6) and H (2.1). Thus polar species i.e., electrophiles or nucleophiles are unable to attack these bonds under ordinary conditions.

Inspite of less reactive nature, alkanes show some characteristic reactions.

(1) Oxidation: Oxidation of alkanes gives different products under different conditions.

(a) Complete oxidation or combustion : Alkanes burn readily with non luminous flame in presence of air or oxygen to give CO₂ & water along with evolution of heat. Therefore alkanes are used as fuels.

C_nH_2n+2 + [(3n+1)/2]O₂ → nCO₂ + (n+1)H₂O; ΔH = -ve

CH₃ + 2O₂ → CO₂ + 2H₂O; ΔH = -ve

(b) Incomplete oxidation : Incomplete oxidation of alkanes in limited supply of air gives carbon black and carbon monoxide.

2CH₄ + 3O₂ → 2CO + 4H₂O

CH₄ + O₂ → C + 2H₂O

carbon black

(c) Catalytic oxidation :

(i) Lower alkanes are easily converted to alcohols and aldehydes under controlled catalytic oxidation.

(ii) Higher alkanes on oxidation in presence of manganese acetate give fatty acids.

CH₃(CH₂)_nCH₃ + 3O₂ CH₃(CH₂)_nCOOH

(d) Chemical oxidation : Tertiary alkanes are oxidized to tertiary alcohols by KMnO₄

2. Substitution reactions :

(i) Substitution in alkanes shows free radical mechanism. For mechanism see free radical substitution.

(ii) Following substitution reactions in alkanes are noticed.

(a) Halogenation :

(i) Replacement of H atom of alkane by halogen atom is known as halogenations.

(ii) Halogenation of alkane is made on exposure to
halogen + alkane mixture to ultraviolet light or at elevated temperature.

R-H + X₂ R-X + HX

(iii) The extent of halogenations depends upon the amount of halogen used.

(iv) The reactivity order for halogens shows the
order : F₂ > CI₂ > Br₂ > I₂

(v) F₂ reacts violently even in dark and reaction may be controlled by diluting fluorine with N₂, whereas iodination is very slow and reversible. Therefore iodination is made in presence of HgO or HIO₃ (oxidants which decompose HI)

[CH₄ + I₂ ↔ CH₃I + HI] × 5

5HI + HIO₃ → 3I₂ + 3H₂O

4CH₄ + 2I₂ + HIO₃ → 5CH₃I + 3H₂O

(vi) The reactivity order for H atom in alkane shows the order:

tertiary hydrogen>secondary hydrogen>primary hydrogen> CH₄

(vii) The halogenations is catalysed by dibenzoyl peroxide.

(b) Nitration :

(i) Replacement of H atom of alkane by -NO₂ group is known as nitration.

(ii) Nitration of alkane is made by heating vapours of alkanes and HNO₃ at about 400^oC to give nitroalkanes. This is also known as vapour phase nitration.

CH_4(g) + HNO_3(g) CH₃NO₂ + H₂O

(iii) During nitration, C-C bonds of alkanes are also decomposed due to strong oxidant nature of HNO₃ to produce all possible nitroalkanes.

(iv) The nitration of alkane also shows the order:

T.H. > S.H. > P.H. > methane

(v) The nitration of alkanes follows free-radical mechanism

HONO₂ HO + NO₂

C₃H₇-H + HO → C₃H₇ + H₂O

C₃H₇ + NO₂ → C₃H₇NO₂

(c) Sulphonation :

(i) Replacement of H atom of alkane by -SO₃H is known as sulphonation.

(ii) Lower normal alkanes are not suphonated, but higher normal alkanes show sulphonation (hexane onwards) when heated with oleum (i.e., conc. H₂SO₄) at 400^oC.

C₆H₁₄ + H₂SO₄ → C₆H₁₃SO₃H + H₂O

(iii) Lower members are sulphonated in vapour phase sulphonation.

(iv) The reactivity order for sulphonation is T.H. > S.H. > P.H. Thus isobutene is easily sulphonated as it contains tertiary hydrogen atom.

(v) Sulphonation of alkanes also follows free radical mechanism.

HOSO₃H HO + SO₃H

C₃H₁₃-H + OH → C₆H₁₃ + H₂O

C₃H₁₃ + SO₂H → C₆H₁₃SO₃H

3. Isomerization :

(i) The process of conversion of one isomer into other is known isomerization.

(ii) Straight chain alkanes on heating with AICI₃ + HCI at about 200^oC and 35 atm pressure are converted into branched chain alkanes.

4. Aromatization :

(i) The process of conversion of aliphatic compound into aromatic compound is known as aromatization.

(ii) Alkanes having six to 10 carbon atoms are converted into benzene and its homologues at high pressure and temperature in presence of catalyst.

C₆H₁₄ + 4H₂

6. Pyrolysis :

(i) The decomposition of a compound on heating in absence of air is known as pyrolysis.

(ii) The phenomenon of pyrolysis of alkane is also known as cracking.

(iii) Alkane vapours on passing through red hot metal tube in absence of air decomposes to simpler hydrocarbons. The product formed during cracking depends upon

(a) nature of alkane

(b) temperature and pressure

(c) presence or absence of catalyst

(iv) The ease of cracking in alkanes increases with increase in molecular weight and branching in alkane.

(v) Fission of C-C bonds produces alkane and alkenes whereas fission of C-H bonds produces alkene and hydrogen.

(vi) Presence of Cr₂O₃, V₂O₂, MoO₃ catalyses C-H bond fission and presence of SiO₂, AI₂O₃, ZnO catalyses C-C bond fission.

(vii) The no. of products obtained during cracking increases with increase in molecular weight of alkane undergoing cracking.

(viii)

(ix) Cracking has an important role in petroleum industry. Higher alkanes are converted into lower one (petrol C₆ to C₁₁) by cracking.

5. Dehydrogenation : Alkanes are dehydrogenated on heating in presence of catalyst to produce corresponding alkenes.

C₃H₈ C₃H₆ + H₂